I think unit testing is about testing the observable behavior of the class under test. Therefore there is no need to test private parts as they themselves are not observable. The way you test it is by testing whether the object behaves the way you expect it (which implicitly implies that all private internal states are in order).
The reason for not to be concerned about the private parts is that this way you can change the implementation (e.g. refactoring), without having to rewrite your tests.
So my answer is don't do it (even if technically possible to) as it goes against the philosophy of unit tests.
